The present invention relates to a coextrusion feedblock for making lightweight, rigid thermoplastic pipe and tubing and more particularly to a coextrusion feedblock having adjusting means for controlling wall thickness and uniformity in the extrusion of thermoplastic pipe and tubing.
Thermoplastic pipe made of solid plastics such as polyvinyl chloride (PVC) and graft acrylonitrile-buta-diene-styrene (ABS) is sold in huge quantities for use in water supply, sanitary and storm pipes, as well as many other applications. One great advantage of the thermoplastic pipe is that it is lighter in weight than the pipe it usually replaces or displaces; another advantage is its ease of fabrication into fluid systems. Its structural integrity is good for many applications, but improvement is desired in other applications.
For many applications a desirable pipe is a smooth surfaced, fluid impervious pipe of a thickness to give it good structural integrity but light enough to have the lightweight and ease of fabrication advantages of plastic pipe. A simple, inexpensive and novel apparatus for making such a pipe is the subject matter of this invention.
The pipe made in accordance with the apparatus of the present invention is formed by extruding multiple layers of the thermoplastic. The extruded outer and inner layers are made of solid plastic and comprise the exterior and interior of the pipe, and the extruded center layer is made of a foamable plastic, preferably the same as the outer and inner layers.
The thermoplastic utilized in making the pipe may be ABS, PVC, phenylene oxide polymer, polycarbonate, polyethylene, polypropylene, polystyrene, polybutylene or combinations thereof. The center layer of foamable thermoplastic contains a chemical foaming or blowing agent which is incorporated into the plastic prior to charging the plastic to the extruder. Suitable foaming agents are bicarbonate of soda and citric acid, azodicarbonamide and the N-nitroso compounds. Modern Plastics Encyclopedia, 1975-1976, at pages 127-129, discloses other chemical foaming agents which can be utilized in making foamed thermoplastic materials.
The apparatus employed for carrying out the method of the present invention comprises a plurality of extruders, preferably two, connected to a single extrusion die having multiple passages for receiving and forming the plastic discharged by the extruders. More specifically, the novel aspect of the apparatus of this invention is adjusting means in the single die for controlling the thickness uniformity of the inner layer of the pipe.